perry metzger, who runs a digital security firm, says there need not be a trade-off between privacy and security. how can that be? >> really mostly what we have is a trade-off between security and non-security. people, you know, might want to read the contents of my grocery list, and i don't particularly care about that. but unfortunately, the contents of my grocery list are protected by the same things that protect my bank information, nuclear power plants, transit systems. you don't get security for one of these things without security for all of them. it is not possible to put back doors into our systems and still have those systems be safe from people with bad intentions. john: why? >> unfortunately, we don't know how to do it. john: is it weird that the candidates rarely get asked about this or speak about it? >> well, it seems to me to be something that should be a major issue. the infrastructure of the united states now runs on computer networks and computers, and disrupting that infrastructure is probably something that people who don't like us might want to be able to do. and y